package Tree;

sub add_item {
   my ($self, $item) = @_;
   my $parent_node = ...;
   $parent_node->add(new Node($item));
}

##</code><code>##

package Tree;

sub add_item {
   my ($self, $item) = @_;
   my $node_factory = $self->{node_factory};
   my $parent_node = ...;
   $parent_node->add($node_factory->create($item));
}

sub set_node_factory {
   my ($self, $node_factory) = @_;
   $self->{node_factory} = $node_factory;
}

package NodeFactory;

# Overridable
sub create {
   shift(@_);
   return Node->new(@_);
}